The proposed research is based on results during the first grant year on BALB/c mice involving the institution of changes in lighting regimen as a major synchronizer. These changes were instituted with various frequencies with varying directions (advances vs delays) and at different ages. The first finding bears on BALB/c female retired breeders 17 months old, housed 5/cage with food and water freely available, one group (I) on a fixed regimen of light and darkness alternating at 12-h intervals, while two other groups were subjected to shifts of this regimen instituted as advances of 6 hours (plus 90 degrees) every 3 days (II), or every 6 days (III). These shift schedules were continued for 35 weeks. survival times differed between the group II and III mice, all shifted late in life and group I) controls (t equals 1.90; P approximately .05; 1-tail). Further work is to investigate lifespan and circadian rhythms in rodents subjected to repeated shifting of synchronizers (daily lighting and/or feeding schedules). Possible contributory factors to be examined are 1) heredity; 2) age at first exposure; 3) interaction of synchronizers; 4) frequency of shifting; 5) extent of shift; and 6) direction of shift. These studies on a few well-controlled models relate to possible adverse or beneficial effects of shift-work in human beings.